Download Oxidative Addition

Oxidative Addition
•
We will now look at a general method for simultaneously introducing pairs of anionic ligands, A
and B, by the oxidative addition of an A−B molecule such as H2 or CH3‐I.
•
The reverse reaction,
Th
ti
reductive
d ti elimination,
li i ti
l d to
leads
t the
th extrusion
t i off A−B
A B from
f
an M(A)(B)
complex and is often the product‐forming step in a catalytic reaction.
•
In the oxidative addition direction, we break the A−B bond and form an M−A and an M−B bond.
•
The oxidation state (OS), electron count (EC), and coordination number (CN) all increase by two
units during the reaction.
•
It is the change in formal oxidation state (OS) that gives rise to the oxidative and reductive part
of the reaction names.
names
1
•
Oxidative additions proceed by a great variety of mechanisms,
mechanisms however,
however a vacant 2e
site is always required on the metal.
•
We can either start with a 16e complex or a 2e site must be opened up in an 18e
complex by the loss of a ligand producing a 16e intermediate species.
•
The change in oxidation state means that the starting metal complex of a given
oxidation state must also have a stable oxidation state two units higher to undergo
oxidative addition (and vice versa for reductive elimination).
2
Binuclear oxidative addition
•
Each of two metals change their oxidation states, electron count, and coordination number by
one unit each.
A
2LnM
+
oxidative addition
LnM
A
+
LnM
B
LnM
A
+
LnM
B
B
A
LnM
MLn
+
oxidative
id ti addition
dditi
B
OS = +1
EC = +1
CN = +1
•
This typically occurs in the case of a 17e complex or a binuclear 18e complex with an M−M
bond where the metal has a stable oxidation state more positive by one unit.
•
f off two electrons into the σ‫ כ‬orbital of the
Whatever the mechanism, there is a net transfer
A−B bond, and the two A−B σ electrons are divided between both metals.
•
This cleaves the A−B bond and makes an M−A and an M−B bond.
3
Change in d n
configuration
Change in coordination geometry
d 10 d 8
Linear
X2
Square planar
Au(I)
Au(III)
11
Tetrahedral
-2L, X2
Square planar
Ni(0)
Pt(0)
Pd(0)
Ni(II)
Pt(II)
Pd(II)
10
10
10
Octahedral
Pt(II)
Rh(I)
Fe(0)
Pt(IV)
Rh(III)
Fe(II)
10
9
8 rare
Octahedral
Co(I)
Ru(0)
Co(III)
Ru(II)
9
8
2 Octahedral
2 Co(II)
2 Co(III)
9 binuclear
2 Octahedral
2 Co(II)
2 Co(III)
9 binuclear
7-coordinate
Re(I)
W(0)
V(-I)
Re(III)
W(II)
V(I)
7
6
5
2 Octahedral
2 Cr(II)
2 Cr(III)
6 binuclear
2 Octahedral
2 Cr(II)
2 Cr(III)
6 binuclear
8-coordinate
Mo(II)
Mo(IV)
6
d8 d6
Square planar
Trig. bipyramidal
d7 d6
2 Sq. pyramidal
2 Octahedral
d6 d4
Octahedral
d4 d3
2 Sq. pyramidal
2 Octahedral
d4 d2
Octahedral
d2 d0
Various
X2
-L
L, X2
X2
-2L, X2
-L, X2
X2
-2L, X2
X2
Examples
Group
5&4
4
•
As we have seen, oxidative addition is the inverse of reductive elimination and vice versa.
•
IIn principle, these reactions are reversible, but in practice they tend to go in the oxidative or i i l th
ti
ibl b t i
ti th t d t
i th
id ti
reductive direction only.
The position of equilibrium in any particular case is governed by the overall thermodynamics
Relative stabilities of the two oxidation states
g
Balance of the A−B vs. the M−A and M−B bond strengths
•
Alkyl hydride complexes commonly undergo reductive elimination of an alkane, but rarely does oxidative addition of alkanes occur.
does oxidative addition of alkanes
occur
•
Conversely, alkyl halides commonly undergo oxidative addition, but the adducts rarely reductively eliminate the alkyl halide.
•
Rare e amples of eq ilibri m do e ist b t are thermod namicall controlled
Rare examples of equilibrium do exist, but are thermodynamically controlled:
5
•
Oxidative addition is usually favored by strongly donating ligands (electron rich substrates)
because these stabilize the increased oxidation state of the central metal.
•
While the change in formal oxidation state is always +2 (apart from binuclear oxidative
addition), the real charge on the metal changes much less because ligands A and B do not
end up with pure −1 charges in LnM(A)(B) – electroneutrality principle !
•
The reall change
Th
h
i charge
in
h
att the
th metal
t l and
d ligands
li
d depends
d
d mostly
tl on the
th
electronegativity of A and B so that the following reagents are more oxidizing in the order:
H2 < HCl < Cl2
•
a
We can estimate the oxidizing power of different reagents experimentally by measuring
ν(CO) on going from IrCl(CO)L2 to Ir(A)(B)Cl(CO)L2 because a more oxidizing reagent will
reduce M−CO back bonding.
reagentt
v(CO)
(CO) (cm
( -11)
1)
 (CO) (cmv(CO)
(
none
1967
0
O2
2015
48
D2 a
2034
67
HCl
2046
79
MeI
2047
80
C2F4
2052
85
I2
2067
100
Cl2
2075
108
The D isotope is used because the Ir-H stretching vibrations have a similar frequency to v(CO)
OC
Cl
L
L
Ir
+
A
B
L
OC
Cl
Ir
B
L
A
6
•
Oxidative additions are very diverse mechanistically therefore we will consider each reaction mechanism independently.
each reaction mechanism independently.
1. Concerted, or three‐center, oxidative addition mechanism
2. SN2 mechanism
3. Radical mechanisms
4. Ionic Mechanisms
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1. Concerted Mechanism
•
Concerted (aka three‐center) oxidative addition is really an associative reaction in which the incoming ligand first binds as a σ complex and then undergoes bond breaking as a result of ‫ כ‬orbital.
strong back donation from the metal into the σ
g
•
Non‐polar reagents, such as H2, or compounds containing C−H and Si−H bonds all tend to react via a σ complex transition state (or even an intermediate) of this type.
•
The associative step a
The
associative step a involves formation of a σ complex; sometimes this is stable isolable involves formation of a σ complex; sometimes this is stable isolable
intermediate and the reaction stops here.
•
Step b is the oxidative part of the reaction in which metal electrons are formally transferred to the σ‫ כ‬orbital of A−B.
8
•
There are many examples of concerted oxidative addition, however, one of the most‐studied
cases is the addition of H2 to the 16e square planar d8 species IrCl(CO)(PPh3)2 [aka Vaska’s
complex ] to give the 18e d6 octahedral dihydride IrCl(H2)(CO)(PPh3)2 .
•
Normally two ligands that are trans in the Ir(I) complex fold back to give the cis dihydride
isomer, but subsequent rearrangement can occur.
•
Conversely, in a reductive elimination such as the loss of H2 from the dihydride, the two ligands
to be eliminated normally have to be cis to one another.
(consider the reverse of the reaction above)
9
•
The C−H and Si−H bonds of various hydrocarbons and silanes can also oxidatively add to
metals.
metals
•
Among different types of C−H bonds, those of arenes are particularly prone to oxidative
addition because of the high thermodynamic stability of the aryl hydride adduct.
•
Agostic complexes,
complexes σ complexes of C−H
C H bonds,
bonds can be thought of as lying along the pathway
for oxidative addition but arrested at different points.
•
A study of the structures of a series of these complexes allowed the kinetic pathway and
j
to be mapped
pp out.
reaction trajectories

The C−H bond seems to approach with the H atom pointing toward the metal.

The C−H bond then pivots around the hydrogen to bring the carbon closer to the metal
in a side‐on arrangement,
arrangement followed by C−H bond cleavage.
cleavage
10
Crabtree et al. Inorg. Chem. 1985, 24, 1986‐1992
11
•
Carbon–carbon bonds do not normally oxidatively add, but a
classic early case,
case the reaction of cyclopropane with Pt(II) to
give a Pt(IV) metalacyclobutane shown below, illustrates how
the reaction can be driven by ring strain.
•
Similarly, biphenylene readily reacts in the same way.
12
•
Upon addition of O2, Ir(I) reduces the O2 to O22−, the peroxide ion, which coordinates to Ir(III).
•
Why
h not envisage the
h reaction as a ligand
l
d addition
dd
b regarding
by
d
O2 as a 2e donor
d
such
h as
ethylene?
 This is a result of the different formal oxidation states assigned to the L and X2 extreme
pictures of binding.
binding

In fact, ethylene is much closer to the L extreme while O2 is very close to the X2 extreme.
 This means that the conventional descriptions of ethylene binding as a ligand addition and
of O2 binding as an oxidative
o idati e addition are the most appropriate.
appropriate
•
For ligands, such as CF2=CH2, which bind in a fashion that is approximately equidistant between
the two extremes, there is a gray area in which the choice between the two descriptions is
arbitrary.
•
This emphasizes that categories such as “oxidative addition” are mental constructs and have
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their limitations.
2. SN2 Mechanism (non‐concerted)
•
In all oxidative additions, a pair of electrons from the metal is used to break the A−B
bond in the reagent.
•
In the SN2 pathway, adopted for polarized A‐B substrates such as alkyl halides, the
metal electron pair of LnM directly attacks the A–B σ‫ כ‬orbital by an in‐line attack
at the least electronegative atom (where σ‫ כ‬is largest) formally to give LnM2+ , A−,
and B− fragments (ionic model).
model)
•
The SN2 mechanism is often found in the addition of methyl, allyl, acyl, and benzyl
halides.
•
Like the concerted type, they are second‐order reactions, but they are accelerated
in polar solvents and show negative entropies of activation (S).
•
This is consistent with an ordered, polar transition state, as in organic SN2
reactions.
14
•
Inversion at carbon has been found in suitably substituted halides.
•
The stereochemistry at the carbon of the oxidative addition product was
determined by carbonylation to give the metal acyl followed by methanolysis to
give the ester.
ester
•
Both of these reactions are known to leave the configuration at carbon
unchanged, and the configuration of the ester can be determined unambiguously
f
from
the
h measured
d opticall rotation off the
h final
f l organic product.
d
15
•
R and X may end up cis or trans to one another in the final product, as expected for the recombination of the ion pair formed in the first step.
vs. concerted mechanism
16
•
Of the two steps, the first involves oxidation by two units but no change in the electron count (Me+ is a 0e reagent)
•
The second step involves an increase by 2e in the electron count (I− is a 2e reagent) but no change in the oxidation state.
•
Only the two steps together constitute the full oxidative addition.
•
When an 18e complex is involved, the first step can therefore proceed without the necessity of losing a ligand first
of losing a ligand first.
•
Only the second step requires a vacant 2e site.
17
•
The more nucleophilic the metal, the greater its reactivity in SN2 additions, as illustrated by the reactivity order for some Ni(0) complexes:
the reactivity order for some Ni(0) complexes: Ni(PR3)4 > Ni(PAr3)4 > Ni(PR3)2(alkene) > Ni(PAr3)2(alkene) > Ni(cod)2
•
(R = alkyl)
Steric hindrance at carbon slows the reaction, so we find the reactivity order:
Me‐I > Et‐I > iPr‐I
•
A better leaving group accelerates the reaction, which gives rise to the reactivity order:
R OSO2(C6H4Me) > R‐I > R‐Br > R‐Cl
R‐OSO
Me) > R I > R Br > R Cl
18
3. Radical mechanism
•
Radical mechanisms in oxidative additions were recognized later than the SN2 and
the concerted processes.
•
They
h can also
l be
b photoinitiated.
h i ii d
•
A troublesome feature of these reactions is that minor changes in the structure of
the substrate, the complex, or impurities present in the reagents or solvents can
change
h
the
h rate, and
d even the
h predominant
d i
mechanism
h i
off a given
i
reaction,
i
i
i.e.
the radical mechanism is highly sensitive to the environment.
•
Sharp disagreements have turned on questions of repeatability and on what types
off experimental
i
l evidence
id
should
h ld be
b considered
id d as valid
lid mechanistic
h i i criteria.
i i
•
For example, the use of radical traps, such as RNO•, has been criticized on the
grounds that these may initiate a radical pathway for a reaction that otherwise
would
ld have
h
f ll
followed
d a non‐radical
di l mechanism
h i in
i the
h absence
b
off trap.
19
•
Two subtypes of radical process are now distinguished:
 non‐chain
 chain
•
The non
The
non‐chain
chain variant is believed to operate in the additions of certain alkyl halides, variant is believed to operate in the additions of certain alkyl halides
R‐X, to Pt(PPh3)3 (R = Me, Et; X = I; R = PhCH2; X = Br).
PPh3
Ph3P
f ast
Ph3P
Pt
Pt
PPh3
PhCH2Br
slow
+ PPh3
PPh3
Ph3P
PPh3
+ PhCH2Br
Pt(0)
Pt(0)
Pt
Pt(I)
Br
Ph3P
CH2Ph
Pt
Ph3P
f ast
Ph3P
Pt
Br
PPh3
+ PhCH2
Pt(II)
Pt(I)
20
•
The key feature is one electron transfer from M to the R‐X σ* orbital to form M+ and (R‐X)•−.
•
After X− transfer to M+, the R•+ radical is liberated.
•
Like the SN2 process, the radical mechanism is faster the more basic the metal, and the more
readily electron transfer takes place, which gives the reactivity order
R‐I > R‐Br > R‐Cl
•
Unlike the SN2 process, the reaction is very slow for alkyl tosylates [e.g., ROSO2(C6H4Me)],
and it goes faster as the alkyl radical, R, becomes more stable and so easier to form, giving
rise to the order of R group reactivity:
3◦ > 2◦ > 1◦ > Me
21
•
The second general kind of reaction is the radical chain.
•
This has been identified in the case of the reaction of Et
This
has been identified in the case of the reaction of Et‐Br
Br and PhCH
and PhCH2Br with the PMe
Br with the PMe3
analog of Vaska’s complex, IrCl(CO)(PMe3)2 .
•
A chain process occurs if the radicals formed escape from the solvent cage without recombination.
•
Otherwise, a radical initiator, Q•, (e.g., a trace of air) may be required to set the process going. This can lead to an induction period (a period of dead time before the reaction starts).
•
• from the halide, to leave the chain ,
,
In either case, a metal‐centered radical abstracts X
carrier R•.
22
•
Chain termination steps limit the number of cycles possible per R•.
2 R → R2
•
The alkyl group always loses any stereochemistry at the α carbon because RR’R’’C• is planar at the central carbon.
•
Unlike the non‐chain case, the reactions slow down or stop in the presence of radical inhibitors, such as the hindered phenol, 2,6‐di‐t‐butylphenol.
•
These quench the radical R• to give R−H and the stable, and therefore unreactive, aryloxy
radical, ArO•.
•
Binuclear oxidative additions, because they involve 1e rather than 2e changes at the metals, often go via radical mechanisms.
23
4. Ionic mechanism
•
Hydrogen halides are often largely dissociated in solution, and the anion and proton tend to add to metal complexes in separate steps.
Two variants have been recognized:
1.
In the more common one, the complex is basic enough to protonate (oxidative ligation), after which the anion binds to give the final product.

2.
protonation – anionation
Rarer is the opposite case in which the halide ion attacks first, followed by protonation
((oxidative ligation)
g
) of the intermediate.

anionation  protonation
24
•
In the more common mechanism of protonation followed by anionation, the complex is
basic enough to protonate, after which the anion binds to give the final product.
PPh3
Pt
Ph3P
Cl
+ HCl
PPh3
PPh3
Ph3P
-2PPh
2PPh3
Pt
H
Ph3P
Pt(II)
Pt(0)
H+
Cl-
-PPh3
-PPh3
PPh3
Ph3P
Pt
H
Ph3P
Pt(II)
•
This route is favored by

polar solvents

g
basic ligands

a low‐oxidation‐state metal
25
•
Like the concerted and SN2 mechanisms the ionic mechanism is second order in rate. Protonation is the rate determining step:
Protonation is the rate determining step:
Rate = k [complex][H+]
•
This can be carried out independently by using an acid with a non‐coordinating anion.
•
HBF4 and HPF
and HPF6 are the most often used.
are the most often used
•
The anion has insufficient nucleophilicity to carry out the second step, and so the intermediate can be isolated!!
•
This is an example of a general strategy in which a “non‐coordinating” anion is used to Thi
i
l f
l t t
i hi h “
di ti ” i i
dt
isolate reactive cations as stable salts.
26
•
Rarer is the case of anionation followed by protonation where the halide ion attacks first, followed by protonation of the intermediate.
•
This route is favored by •

polar solvents

electron‐acceptor ligands

a net positive charge on the complex
h
h
l
Polar solvents encourage both ionic mechanisms – why?
27
•
Again this is a second order reaction but now follows a rate dependent upon the counterion
concentration as X addition is the rate limiting step.
concentration as X
addition is the rate limiting step
Rate = k [complex][Cl]
•
Similar to the first type mechanism, this step can be carried out independently with LiCl
alone.
•
No reaction is observed with HBF4 alone!

because the cationic iridium complex is not basic enough to protonate and BF4− is a non‐
coordinating anion.
28